Abstract

Organic production systems are possible alternatives to current conventional dryland cropping systems in south western Australia, but they remain largely untested for their impact on crop production and soil fertility. The aims of this study were to compare wheat production levels of organic and conventionally managed fields, and relate outcomes to possible differences in soil fertility. Seven paired sites were compared over three years (1992–1994); the sites were paired to ensure similarity in soil types, crop history before conversion of the organic paddock from conventional to organic farming, and at most sites, the farm manager of paired fields was the same. In years 1 to 2, there was no difference between the following soil chemical properties of the paired organic and conventionally managed fields: pH, electrical conductivity, mineral nitrogen (ammonium-N and nitrate-N), soil organic C, total P and lactate-extractable-P.

Compared to the grain yield of the conventionally grown wheat, yield of the organically grown wheat was: increased at one site by 17%; decreased at three sites by an average of 27%, and not changed at another three sites. Several factors were considered to be most likely causes of the grain yield differences between the organic and conventional wheat: delayed sowing, lower nitrogen supply, and lower Colwell-extractable-P of the organic wheat. The key sustainability issue in the organically managed fields was P supply. In the organically managed fields, 3–10 kg P/ha/crop was removed from the wheat field and it is not currently being replaced. This is likely to eventually result in decreases in grain production due to P deficiency.